1. Field of the Invention
The present invention relates to a preceding vehicle detection system and more particularly to a preceding vehicle detection system which can detect tail lamps of a preceding vehicle.
2. Description of the Related Art
In recent years, the development of technologies has been in progress in which a three-dimensional object lying on the periphery of a vehicle such as a passenger vehicle is detected by analyzing an image captured by a CCD (Charge Coupled Device) camera or analyzing reflected wave of radio wave emitted from a radar system (refer, for example, JP-A-7-225892). These technologies are applied, for example, to technologies for safety driving of vehicles in which the possibility of collision of the vehicle with the three-dimensional object so detected is judged, so as to activate an audible warning system to give the alarm to the driver or activate an automatic steering or automatic brake control to avoid the possible collision.
In addition, various technologies have also been under development of detecting brake lamps and tail lamps of a preceding vehicle from a captured image of what lies in front of a subject vehicle in order to enhance further the safety driving of the subject vehicle so as to prevent the collision thereof with, in particular, the preceding vehicle.
For example, in JP-A-8-241500, a system is proposed in which due to the brake lamps being red and the direction indicator lamps being amber, red areas and amber areas are separately extracted from a color image so as to detect that a preceding vehicle is turning to the left or right, or that the brakes are applied. In addition, in JP-A-11-39597, a system is proposed in which a preceding vehicle is detected by a frame line from an image and a pixel area having a luminance which is 1.5 or more times as high as a luminance average value of pixels within the frame line is detected as tail lamps and brake lamps of the preceding vehicle which are being illuminated.
Incidentally, the technology disclosed in JP-A-8-241500 can be applied to detect the tail lamps of a preceding vehicle during a night-time driving. In the system disclosed in JP-A-8-241500, however, an image capture unit which can be used together with the system is limited to an image capture unit which can capture a color image, and hence, the technology of JP-A-8-241500 cannot be applied to a preceding vehicle detection system which is equipped with only an image capture unit which captures an image which is represented as gray scale values. In addition, since colored light cannot be distinguished, light rays from the tail lamp, brake or stop lamp, turn signal lamp and license plate lamp cannot be detected discriminately by the color of light emitted therefrom.
On the other hand, with the preceding vehicle detection system disclosed in JP-A-11-39597, the tail lamps of a preceding vehicle can be detected based on an image captured in gray scale. In addition, in this system, the tail lamps of the preceding vehicle are detected based on the surface areas or positions on the image of two highly bright regions which are considered to result from the illumination of the tail lamps.
However, as is shown in FIG. 21, which is described later, for example, in a case where an oncoming vehicle Vonc is detected at the side of a vehicle ahead or preceding vehicle Vah, since a pixel area where light Ltail of a tail lamp of the preceding vehicle Vah is captured and a pixel area where light Lhead of a headlamp of the oncoming vehicle Vonc is captured are integrated with each other on the image and is hence captured as a highly bright area, the tail lamp and the headlamp cannot be discriminated from each other, and a difference in area between left and right bright areas exceeds a permissible difference or the left and right bright areas are judged as being positioned differently on the image, whereby there has occurred a case where the tail lamp of the preceding vehicle cannot be detected effectively or the detection thereof becomes unstable.